1. Field of the Invention
The present invention relates generally to a semiconductor wafer testing apparatus. More particularly, it relates to an improved semiconductor wafer testing apparatus which has an automated station in which both wafer alignment and wafer recognition are performed under automatic control before a wafer test, without having to move the wafers to two separate stations.
2. Background of the Related Art
For each step in semiconductor device fabrication, a testing apparatus is used to inspect and measure the devices upon completion of the corresponding process. Generally, one wafer that has already undergone a prescribed process of fabrication is preselected to be placed under sampling inspection in the semiconductor wafer testing apparatus.
The sampling inspection procedure of the conventional wafer testing apparatus is now described with reference to FIG. 1.
The sampling inspection begins with loading a carrier full of semiconductor wafers which have undergone a prescribed process onto a wafer alignment mechanism such as a flat zone aligner 11 provided in the semiconductor wafer testing apparatus. There, the semiconductor wafers are aligned such that the flat zone of each wafer faces a given direction. The wafer alignment mechanism is generally either a three-dimensional coordinate type or a roller type, of which the three-dimensional coordinate type is coming into more common use. The alignment is performed by rotating each wafer until a photosensor senses the flat zone of each wafer, at which time the rotation of the wafers stops.
The carrier containing the aligned wafers is then placed on an identification (ID) finder 12. The ID finder 12 is used to identify the semiconductor wafers. The ID finder 12 arranges the wafers in tiers, which makes it easy to see wafer codes which are located on an upper portion of the flat zone of each wafer, and to thereby find a desired one of the wafers. The wafer identification is necessary because the sampling inspection will be made on a wafer with a predetermined wafer code that is preselected from a lot of wafers, and the wafer with the predetermined number is not always placed in the same spot within the carrier during the sampling inspection.
Once the operator finds a wafer 10 with the predetermined wafer code, he removes it from the carrier with tweezers and puts it into another carrier which is laid on a carrier stage 13. Then, the operator works the keyboard of a controller 14, and according to the keyboard input, an autoloader 15 of the wafer testing apparatus takes the wafer from the carrier stage 13 and places it onto a stage of a microscope 16. The operator checks to see if there is any defect in the wafer by the use of a monitor 18 connected to a microscope camera 17. Following the inspection, the wafers are again loaded on the original carrier in the reverse order for subsequent processing.
According to the conventional wafer testing apparatus, the flat zone aligner and the ID finder are needed to locate the preselected wafer for testing, and the operator must visually select the wafer with the predetermined wafer code and move it to the carrier stage by hand. During this processing, as the operator looks closely at the wafer to see its code, contamination from the operator may be generated as well as particulate dust due to the use of the tweezers, which contamination may cause problems in subsequent processing.
Accordingly, several methods of automatically recognizing a wafer with a predetermined wafer code have been proposed in order to solve this problem.
Of the recently-developed methods, one employs optical character recognition. An optical character recognizer is used with presently-available wafer sorters. The wafer sorter is used to arrange the wafers that are mixed up in the carrier.
Referring to FIG. 2, a system using an optical character recognizer is now described hereinafter.
A first carrier 21 in which wafers 20 are mixed up after processing is placed on one side of a work table 28 of the wafer sorter, with the wafers in a horizontal position. An autoloader 22 installed on the center of the work table 28 picks up the wafers one by one out of the first carrier 21 and puts them on a spin chuck 23 of the work table 28. The spin chuck 23 rotates and allows the wafers to be aligned in order that their flat zones face a given direction by using a sensor 24 for sensing the flat zone of each wafer. This alignment mechanism is substantially similar to the conventional three-dimensional coordinate type.
Then an optical character recognizer 25 reads out the respective codes on the flat zones of each wafer and sends the codes to a controller 26. The controller 26 then drives the autoloader 22 to convey the wafers to slots of a second carrier 27, with slot placement corresponding to wafer codes of each wafer.
When the optical character recognizer used for such a wafer sorter is employed in a semiconductor wafer testing apparatus, wafer recognition must wait until the wafers have been conveyed one at a time from the carrier to the spin chuck for wafer alignment. Accordingly, a lot of time is required for completion of the overall processing.